Turns-counting dials have been commercially available for many years in a variety of styles, and typical units are disclosed in U.S. Pat. Nos. 2,746,417--McCord, 2,805,636--Smith, and 3,450,091-Wajdik. These dials are used to display the angular position of multi-turn devices such as potentiometers which may be rotated through, for example, 10 full turns.
A knob of the dial is secured to a shaft of the device, and a "fraction" scale on the knob displays shaft position from 0.degree.-360.degree. (the scale is commonly graduated in decimal form to show 100 counts for one full revolution). A second "turns" scale is mechanically coupled to the knob, and is graduated in the number of turns to be counted. The coupling mechanism moves the turns scale by one count for each full rotation of the knob. The two scales thus display the total rotational displacement of the shaft from a zero or starting position.
The coupling mechanism in these dials has several functions. First, the mechanism rotates the turns scale through an angle equal to one count on the scale for each rotation of the knob. Second, the mechanism locks the turns scale against inadvertent rotation arising from ambient vibration, or from manipulation of the scale by the operator.
In the dials disclosed in the aforementioned Smith and Wajdik patents, a Geneva mechanism is used to effect intermittent coupling between the knob and turns scale. An annular shoulder on the knob (or an intermediate drive ring) is positioned adjacent the periphery of the Geneva indexing wheel to lock the wheel and turns scale during dwell operation between once-per-rotation advance movements.
The McCord patent shows another type of coupling mechanism using an intermittent-tooth "mutilated" gear having teeth which are alternately of full and partial width. All teeth on this gear are meshed with a turns-scale gear, and an annular locking shoulder on a locking ring positioned radially outward of the mutilated gear engages the tips of the full teeth to lock the turns scale except when the gear is engaged by an inwardly facing gear segment during once-per-turn advance motion.
The dial of this invention has an improved intermittent-motion mechanism which facilitates dial miniaturization which is important where the dial must be mounted on a very small panel, or in applications requiring the mounting of a large number of dials in a limited space. The improved mechanism also provides secure mechanical locking of the turns dial without requiring shielding of the turns dial against operator access to protect the relatively fragile coupling devices used in earlier designs. The entire surface of the turns scale can thus be made visible to the operator to eliminate reading error during transitional movement of the turns scale. This has been found to be an important feature in turns-counting dials, but has been avoided in some prior-art devices to prevent inadvertent torquing of the turns scale (either in regular use of the dial, or particularly during installation of the dial on a panel-mounted potentiometer or similar device) which could distort or break the coupling mechanism. The new design is also readily modified to count various numbers of turns, or to advance the turns scale for fractional rotation of the control knob.